The resin transfer molding (RTM) process has become a popular composite manufacturing process due to its suitability for high volume production and cost effectiveness. In this process, the dry fiber reinforcement (preform) is enclosed in the mold and resin is injected and allowed to cure [see reference 1].
The impregnation of the fluid resin into the preform is defined by the permeability of the reinforcement, which is the ability of a Newtonian fluid to permeate a porous medium with a sufficiently low Reynolds number, as given by Darcy's Law. The permeability of an undistorted preform with a constant fiber volume ratio can be assumed to be uniform over the entire domain; however, the permeability can be significantly altered by defects, distortions, or other anomalies in the preform. Such drastic changes in local permeability can affect resin flow patterns, rendering portions of the mold to be insufficiently filled. Permeability variations within a preform can be attributed to a number of factors, such as improper preform preparation, misplacement or shifting in the mold, accidental inclusion of foreign material, natural surface density variation of the preform, etc. If such occurrences were not detected prior to resin injection, the potential for costly part scrapping would be increased [see references 2 and 3]. Aside from the additional voids due to permeability non-uniformities, preform distortions could contribute to residual stresses and stress concentrations during in-service loading. Early detection, therefore, of such reinforcement irregularities is critical for effective quality control.
Conventional methods for permeability measurement rely on oils or other viscous fluids to be injected into the reinforcement. These fluids soil the fibers and cannot be removed without damage to the preform. Furthermore, the bulk material permeability data obtained from such tests would not provide the information regarding local permeability variation, which is necessary for locating defects. Such strategies, therefore, could not logically be applied in-situ prior to an RTM run. Methods, however, providing multiple pressure measurements with a gas, like the gas flow method, are well suited for such applications.
Published U.S. Patent Application US 2002/0046596 A1 describes a system for in-situ and on-line monitoring of a preform quality for liquid composite molding using flow of a gas through the preform in the mold cavity. Pressure transducers communicated to openings in a lower mold section provided sensed gas pressure values to a data acquisition and processing device having software which generates a pressure profile of the gas flow for the preform. This pressure profile is evaluated by comparing it with a theoretically calculated pressure profile. The method described in the patent application is inherently one-dimensional and was not proven with preform materials having varying degrees of anisotropy. Furthermore, it requires prior determination of the permeability of the preform material.
Liang et al. suggested the possibility of using the gas flow method for defect detection by measuring permeability of a preform with a rectangular cutout region [reference 4].
Copending U.S. application Ser. No. 10/611,318 filed Jul. 1, 2003, by Daniel, Kim, and Opperer describes a gas flow method for detecting and characterizing defects associated with a fiber prefrom using steady state or constant flow of gas through the preform in the mold cavity [also see references 2-3]. In this steady state gas flow method, pressure data are obtained at a plurality of ports communicated to the mold cavity during steady state gas flow. The pressure data is statistically analyzed by discriminant analysis and compared with reference data from a similar gas flow test using a good (high quality) preform. Each steady state test generates a single data point for a partcicular preform for the comparison. The magntiude of deviation of a preform from the reference data is used to determine whether the preform is defective or not.